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How Earth seems quite unique

  1. Jun 27, 2009 #1
    What single observable (such as complexity) might characterize Earth as unlike any other planet in the universe?
     
  2. jcsd
  3. Jun 27, 2009 #2
    The large moon, stabilizing it's spin axis, preventing Earth from getting into the chaotic zone
     
  4. Jun 27, 2009 #3

    Integral

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    Since we have observed such a tiny percentage of planets in the universe we can only speculate about how unique we are.
     
  5. Jun 27, 2009 #4

    Xnn

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    There's roughly 10^24 planets in the universe.

    No reason to suspect that ours is unique.
     
  6. Jun 27, 2009 #5

    turbo

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    And some reason to suspect that all of them are unique in some way. We have a very small sample to observe here in our neighborhood, and there are some vast differences between them in many metrics. There is no reason to assume that other planetary systems would not have significant differences, as well. We have discovered extra-solar planets by lots of means, not the least of which is tracking light-curves to discover large planets partially occulting their stars when their orbits are aligned with our line-of-sight. Are we going to find a "Mercury" orbiting a large star that way? Probably not for a very long time.
     
  7. Jun 27, 2009 #6
    Are you sure this number isn't a substantial overestimate? First start with the visible universe. Take 10^11 stars per galaxy and say 5x10^11 galaxies (which would be much more than the ones we can see). So that's 5x10^22 stars. How many of those stars have planets? Most stars we know of are multiple star systems which are unlikely to have planets. It seems you might be over-estimating by a factor of at least 10^3. We really don't have enough information to estimate the number of planets in our galaxy, let alone the universe. The inner galaxy is a very different neighborhood compared to our location out in one of the spiral arms. I grant you, there are very likely many, many planets in the universe, but I wouldn't try to put a number on it. Estimates for Drake's equation are deliberately kept very conservative.
     
  8. Jun 27, 2009 #7
    Might such a large moon - having its genesis in the coalescing debris of a planetary impact - be more likely suited to avoid chaos by stabilizing the spin axis of the resultant planet?
     
  9. Jun 27, 2009 #8

    Xnn

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    You've missed the point...

    Consider it 10^20+
     
  10. Jun 27, 2009 #9

    mgb_phys

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    Earth has both cyrus and evo - that must be pretty unique!
     
  11. Jun 28, 2009 #10
    Good one. Would every spinning (teristrial?) planet without substantial moon eventually enter the chaotic zone or not? This is caused by -as Laskar proposes- resonance between the obliquity cycle and the precession cycle. The moon speeds up the frequency of the precession cycle, preventing resonance, but what does it do to the obliquity cycle?

    A good thesis question?

    Second question of course is, how unique is that moon? Regardless of which hypothesis is true for the forming of Earth's moon, how feasible are these mechanims for forming similar planet - moon pairs elsewhere?
     
  12. Jun 28, 2009 #11

    Integral

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    I am not sure what the deal is with the moon and stability? We have moonless Venus and Mercury, clearly a moon is not a necessary thing. Still we simply do not have the knowledge to talk about what is common or uncommon in the universe in general.
     
  13. Jun 28, 2009 #12

    D H

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    Read section 2.2 of the paper that Andre cited. It is Earth's obliquity rather than its orbit that the Moon acts to stabilize. Lasker is claiming that Venus' rather anomalous rotation is due solely to gravitational effects by the other planets. The Moon acts to stabilize the Earth's rotation.
     
    Last edited: Jun 28, 2009
  14. Jun 28, 2009 #13

    negitron

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    Without a large, stabilizing moon, Mercury, Venus and Mars all experience much larger variations in axial obliquity--Venus has turned itself nearly completely upside down! It's unclear as to whether a relatively stable axis is necessary for life but what is clear is that things here would be very different without our Moon. For one thing, seasonal variations would be far more extreme.
     
  15. Jun 28, 2009 #14
    But are Venus and Mercury anywhere near Earthlike? The only thing that Venus shares with Earth is the order of magnitude of its size

    And about the (r)evolution of it's rotation:

    Correia A.C.M., Laskar J., de Surgy O.N. (2003). "Long-term evolution of the spin of Venus: I. theory" (PDF). Icarus 163: 1–23
    ^ Correia A.C.M., Laskar J. (2003). "Long-term evolution of the spin of Venus: II. numerical simulations" (PDF). Icarus 163: 24–45

    which elaborates extensively that the lack of rotation of Venus is due to the chaotic zone, resonance between the obliquity cycle and precession cycle, causing excessive tilting of the spin axis, ultimately bringing the rotation of the planet to a halt. It's supposed to be the moon preventing the same fate for Earth.
     
  16. Jun 28, 2009 #15

    negitron

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    No, but that wasn't at all my point.
     
  17. Jun 28, 2009 #16
    But it is the subject of the thread.
     
  18. Jun 28, 2009 #17

    negitron

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    No, it's not.
     
  19. Jun 28, 2009 #18

    D H

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    That was pithy. :rolleyes:

    The subject of this thread is the uniqueness of the Earth. Loren asked
    That was not the best way to phrase the question (in fact, I don't quite know what question Loren is asking here). Suppose the Earth does have a near twin somewhere in the universe, right down bearing humaniform intelligent life capable of sending spacecraft outside the planet's gravitational sphere of influence. Something will certainly distinguish our Earth from that near twin, even if it is milligrams in difference in planet mass or centimeters in difference in orbital distance from the parent star. Suppose the Earth has lots of such near twins. One may differ in mass, another will orbit at a slightly different radius, another will orbit a star with slightly different characteristics. There might be a lot of planets out there, but it is a finite number. Our Earth will always differ from its near-twins in some regard.

    I don't think that is what Loren was asking for. Asking for one single characteristic was part of the problem. Those who espouse the rare Earth hypothesis claim that the Earth's (relative) uniqueness results not from one characteristic but a whole slew of characteristics.


    That said, my answer to Loren's original question is that the Earth supports intelligent lifeforms capable of sending spacecraft outside the planet's gravitational sphere of influence.

    Is this characteristic unique throughout the universe? I doubt it. Whether it is rare is an entirely different question. If the closest planet to Earth that bears this characteristic is hundreds of thousands of galaxies away, does it really matter that we aren't unique? We are utterly, utterly alone. Does it really matter if the closest intelligent life-bearing planet is a lot closer, say ten galaxies away? Even in that case, we are still utterly alone, even though the Earth is far from "unique".
     
  20. Jun 28, 2009 #19
    The subject here is really whether there are any other planets having characteristics friendly to intelligent life as we know it. Any planet that we are likely to ever discover can be expected to be unique in some way. Key points:

    .If the planet is beyond any future limit of human exploration or discovery (whatever that might be), the question is irrelevant. However, "they" might be able to find us, in which case they might be kind enough to tell us about their home planet. Then again, they might not be so kind.

    .There might civilizations that have long since disappeared (if and when we find evidence of them) or that have flourished (or are flourishing) in the unseen void where the light has not yet reached us.

    .We might well find planets with life, but not intelligent life; which I take to mean having some kind of technology that we would recognize. These (non-intelligent) possibilities exist in our own solar system: on the Jupiter's moon Europa (covered with water ice, but with a vast liquid sea below), or the Jovian atmosphere at a level where liquid water exists at earth-like temperatures and pressures.

    .If we do discover a "twin earth" we might be way too early (or too late) to shake hands (or whatever) with an intelligent species. After all, intelligent life (as I defined it) is rare on earth. In the case of a technologically advanced species (which I define as being able to use electromagnetic devices for communication), this has existed on earth a very short time; since about 1844 compared to the 4.6 billion years that Earth has existed.

    Note: I've plagiarized much of this post from my own book. (see my member page if interested).
     
    Last edited: Jun 28, 2009
  21. Jun 28, 2009 #20

    D H

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    I agree that that is the hidden agenda behind his original post. That is not what Loren asked in the original post. What Loren asked was "What single observable (such as complexity) might characterize Earth as unlike any other planet in the universe?"

    This is a silly question that verges on being a straw man. Only the most vehement of rare Earth proponents say the Earth is "unlike any other planet in the universe." Does it really matter whether there are hundreds of thousands versus just one Earth-like planets spread around the universe? The end result is the same: We are alone. The only way we are not alone is if there are hundreds of billions of Earth-like planets spread around the universe.
     
  22. Jun 28, 2009 #21
    You are probably right in terms of accessibility at least. But you can't assert this as fact. You need to inject just a little bit of uncertainty into your statement.
     
  23. Jun 28, 2009 #22
    Please allow me to amend my original question. Replace

    "What single observable (such as complexity) might characterize Earth as unlike any other planet in the universe?"

    with the more feasible

    "How might Earth's physics be most unlike that of any practically observable planet?"
     
  24. Jun 29, 2009 #23
    which brings us back to my original answer: the moon
     
  25. Jun 29, 2009 #24

    D H

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    That's a better question, but you are still eliminating a lot of explanations for why some say the Earth is rare. The word "physics" pretty much eliminates geological, chemical, and biological concerns (assuming you don't consider geology, chemistry, and biology to be branches of physics). The words "Earth's physics" pretty much eliminates attributes of the parent star and perturbing effects by other planets in the star system.

    How about "What scientific characteristics might make the emergence of Earth-like intelligent life on a planet rare or common?"


    Requiring the planet to be observable eliminates the vast majority of the universe. At present, our ability to observe planets is very limited. While this ability will certainly improve a lot, will it ever improve to the point of being able to observe a planet in Andromeda, Messier 87, or beyond?

    The Moon is one possible answer. The most widely accepted hypothesis regarding the Moon is that a Mars-sized object formed in the vicinity of one Earth's triangular Lagrange points. Jupiter pumped energy into the Earth-Theia system, eventually making the two bodies collide. This hypothesis solves several issues surrounding the giant impact hypothesis. A collision with a non co-orbiting planetoid would most likely have imparted too much energy and would have destroyed both the Earth and the body.

    A collision with a non co-orbiting planetoid would constitute a freak event. A collision with a body that formed in the vicinity of one of the triangular points is not so far fetched. The Moon might not be quite the unique feature that some think.


    A better answer is that the culmination of many different features make the Earth relatively rare. Asking for one feature is akin to asking what one feature most distinguishes Obama (or Einstein, or you) from everyone else.

    Among the distinguishing characteristics:
    • Location of the star. A star that is too close to the galactic core will have a greater chance of close encounters with other stars, possibly disrupting the orbits of the planets about the star and will have a greater chance of exposure to calamities such as supernovae. Stars far from the galactic core have low metallicity, possibly too low to form rocky planets.
    • A single star. Most stars are in binaries or even larger groups.
    • A stable star. Some stars exhibit a lot greater variability than does the Sun.
    • Planet location. Venus formed too close to the Sun; Mars, too far.
    • Gas giants. Our Jupiter stopped its sunward migration. This is not the case everywhere; many stars have hot Jupiters. Jupiter helped clear out the junk from the Solar system and may have helped the Earth form.
    • Size. Too small and the planet will cool off too quickly. There are problems with being too big as well.
    • A large Moon. This has been covered already.
    • Water. The Earth's surface is 2/3 water. Could an otherwise Earth-like world that is 100% covered with water have spawned intelligent life capable of moving off of the planet? Could an otherwise Earth-like world that barely has any water have developed life?
    • Life and complex life. We do not know whether life is a fluke or an inevitability. We do know that life was very simple for a long, long time. Even if simple life is a highly probable event, complex life might be not be.
    • Metals. Intelligent life might arise on a planet that has a very limited amount of metal. Could it proceed beyond stone age capabilities?
    • Fuel. Our fossil fuels arose when life went bonkers. A planet on which life just gets by may not have produced the fuels needed to sustain a modern civilization.

    Whether these are truly applicable and make the Earth rare, we don't really know. We might have a better idea after the results from Kepler start coming in. Kepler is only going to answer a small number of these questions.
     
  26. Jun 29, 2009 #25

    Xnn

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    Depends on how one describes earth in the first place. Consider the following:

    A planet within 10% of earths mass, a primarily nitrogen atmosphere with oxygen between 15 - 25%, mostly covered with water, average temperature between 10 to 20C.

    By that definition, Earth is probably 1 in a 10 billion. Pretty rare, but in the visible universe, this would mean there would be billions of other planets that fit the same definition. However, the nearest similar planet would probably be at least a 1000 light years away. Likely too far for any type of communication as we know it due to energy and time requirements.

    Of course, any such earth like planet that does exist would likely have slightly different coastlines, different plants and animals. The intelligent creatures (if they were still alive) would not look exactly like ourselves, and would surely have a different set of languages and history. They would also have different names and lives than each of us.

    However, consider that the visible Universe is not the only Universe in existence. Instead, it is just a single Universe of the greater Multiverse.

    http://en.wikipedia.org/wiki/Multiverse

    In the Multiverse, numerical constraints are no longer and it become possible for parallel universes and of course parallel planet earths. On the parallel earth everything is the same. It’s inhabited by intelligent creatures just like ourselves. The coastline and cities and countries are just as we know them. The speak the same language, have the same names and history and are doing exactly what we are doing right now.
     
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